NSF Young Investigator

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ABSTRACT CTS-9457083 Northwestern University P.I.: Wesley Burghardt The central theme of research in my group at Northwestern in the relationship between flow-induced microscopic structural changes in complex fluids, the resulting mechanical rheological behavior behavior, and further, the consequences of nonlinear rheology in more complex fluid mechanical problems. Under NYI support, we will pursue three projects, (i) Flow-induced alignment in block copolymer solutions. Ordered block copolymers are characterized by complexity at two structural levels. Molecular relaxations are reflected in viscoelastic behavior. However, the self-assembly of the polymers into ordered microstructures (such as lamellae) renders both structure and rheological properties highly anisotropic. We will use flow birefringence to study the development of macroscopic alignment under flow, coupled with mechanical testing to study how bulk rheological behavior is affected by the average orientation state. (ii) Constitutive instabilities in highly entangled polymers. We will investigate two classes of materials where the constitutive behavior has been proposed to be subject to an unstable nonmonotonic dependence of stress on deformation. Highly entangled polymer solutions will be studied in step strain, while so called living polymers (micellar surfactant solutions) will be studied in steady shear flow. In both cases, we will utilize optical approaches developed in our laboratory for studying the complete stress tensor using the stress-optical rule. (iii) Non-newtonian fluid mechanics. Experimental studies will be conducted on well-characterized viscoelastic polymer solutions in complex geometries. These efforts are directed towards developing an experimental database with which to test numerical simulations of viscoelastic flow, and discriminate among competing constitutive models.